and also a group that actually has a decent campaign going* that is pretty interesting* and which everyone actually wants to play for that long* which naturally you would want, since you're currently in a pretty crappy playing situation from the sounds of it
* (presumably, from an outsider's perspective)
Then again you're in another campaign online you're not enjoying very much which you're playing just because you've invested that much time in it, you think you'd actually be losing something by moving on. Or that was the case a couple of weeks ago. These people, 70 levels and 10 years later, could be having the time of their lives... whilst they're anywhere but at the D&D game they're only continuing because they've invested 10 years in it and don't really want to start new characters now.
I've been depressed too at times (probably nothing quite like you, but that doesn't make it any less the case) and it's lovely how one's imagination can work to make it look like everyone else is having a pretty good time just to make you feel like you're missing out on something, or you're no good, etc etc.
In reality you're far better off just concerning yourself with what you want to do, and finding out what to do about getting it. You want a decent D&D game. Go find one. You don't really want to play with your friends, or play this one online - but you've invested a lot of time in it, and feel like you'd be losing something by stopping. What are you losing exactly? That invested time was invested in the past, and what you do now is irrelevant to it.
There's a girl I find rather inspirational I read about in a magazine a couple of years ago. She was in her mid-20s and had completed a bachelor or finance or business or some such and had scored a rather successful job and reached a six figure salary.
Then she decided to pursue another job which would earn her at least twice as much. That job was as... an escort. Relevance to her previous job and education? Very little. That wasn't a reason for her not to do it - a lot of people would think so, but what is she losing exactly?
In the interview, she was already lining up her next job after that (something equally irrelevant) and had a long-term plan for where she wanted to be a few years after that.
All along, simply doing what mattered: making a decent living and enjoying life.
Moral of the story: the only thing anchoring you to what you've done up to this point is nothing at all and your choice.
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